It is known to provide a seat belt retractor wherein a winding spring biases a belt reel in the direction to wind the belt on the reel. The winding spring is conventionally a spiral or clock spring having one end mounted on the reel shaft and the other end mounted on the retractor housing. As the reel is rotated in the belt unwinding direction by belt extension, energy is stored in the spring for biasing the reel in the belt rewinding direction.
It is characteristic of the aforedescribed conventional seat belt retractor that when the belt is fully unwound and engaged in the restraining position about the seated occupant, the winding spring is fully wound up and provides its highest level of winding bias on the reel. This occurs because, as the belt is unwound from its fully wound position, the winding spring is increasingly wound up so that it initially provides a relatively smaller torque on the reel and proceeds to a tightly wound condition wherein a relatively higher biasing torque is provided on the reel so that the tension on the belt increases. Furthermore, as the belt is progressively extended and unwound from the reel, the diameter of belt wound on the reel is reduced and progressively diminishes the moment arm through which the spring torque acts upon the belt so that the tension on the belt increases.
It is known that a relatively high biasing effort on a seat belt tensions the belt against the occupant and may be uncomfortable. It is therefore desirable to provide a winding spring which imparts a relatively modest winding bias on the reel when the belt is unwound to the restraining position so that the belt is not uncomfortably tensioned across the body of the seated occupant. It has been found that a winding spring having this desired characteristic may not be sufficient in strength to completely wind the seat belt on the reel and fully retract the belt from its restraining position to its stored position.